The loss of dentin matrix protein 1 (DMP1) function in humans and mice results in autosomal recessive hypophosphatemic rickets (ARHR), characterized by hypomineralization in dentin and bone, hypophosphatemia and the elevation of circulating fibroblast growth factor 23 (FGF23) levels. The dental and skeletal defects of Dmp1-deficient subjects have been attributed to the combined effects of the intrinsic differentiation defects of Dmp1-null odontoblasts and osteoblasts/osteocytes (local effect) and hypophosphatemia (systemic effect). However, the way in which the loss of DMP1 function causes the differentiation defects of odontoblasts and osteoblasts/osteocytes remains largely unknown. The goal of this application is to identify a nuclear isoform(s) of DMP1 (referred to as nuDMP1) and to study its function in odontoblast differentiation. We hypothesize that nuDMP1 is translated from alternative downstream in-frame AUG start codon(s) and controls odontoblast differentiation by regulating Dspp expression. This hypothesis is based on our recent in vitro and in vivo findings, which include: 1) nuclear DMP1 is observed in cells transfected with a construct expressing full-length DMP1 with the endoplasmic reticulum (ER)-entry signal peptide sequence; 2) a construct expressing full-length DMP1 without the ER-entry signal peptide sequence produces two proteins, a full-length DMP1 and a putative nuDMP1; 3) a construct producing the putative nuDMP1 is more potent at stimulating Dspp promoter activity than those producing full-length DMP1 with or without the ER-entry signal peptide; and 4) the transgenic overexpression of DSPP driven by a 3.6 kb Col1a1 promoter rescues the dental defects but not the other manifestations of Dmp1-null mice. To test our central hypothesis, three specific aims are proposed. Aim 1 will determine the mechanisms by which the nuDMP1 is generated and subsequently translocated into the nuclei. Aim 2 will investigate the function of the nuDMP1 in odontoblast differentiation in vivo by generating and characterizing the 3.6 kb Col1a1-nuDMP1 transgenic mice. Aim 3 will determine how Dspp expression is activated through nuDMP1 and if the failure of this process is solely responsible for the tooth phenotype in Dmp1-null mice. Successful completion of this novel proposal will provide unique insights into how DMP1 regulates cell differentiation via the nuDMP1, which is independent of its function in regulating matrix mineralization. The identification of this novel nuDMP1 and its function will not only help elucidate the molecular mechanisms through which DMP1 controls the formation of tooth and bone but will also provide a scientific basis for developing therapies for treatment of hypophosphatemic rickets.